The 200 seconds of autonomous flight by the U.S. Air Force&apos;s X-51A set a duration record for an aircraft powered by a scramjet (short for “supersonic combustion ramjet”) engine, though in part that can be chalked up to the rarity of any flights at all at this extreme level. The previous record was set by NASA&apos;s X-43A, whose scramjet engine burned for only about 10 to 12 seconds in November 2004; that aircraft zoomed to Mach 9.8.

It had been expected that the X-51A would fly for as long as 300 seconds (that is, five minutes) and would hit Mach 6. During Wednesday morning&apos;s flight, the Waverider suddenly lost acceleration shortly after the 200-second mark for an unexplained reason. “At that point,” Boeing&apos;s press release says tersely, “the X-51A was terminated as planned.”